A conventional electromagnetic actuator for opening and closing a valve of an internal combustion engine generally includes an electromagnet having a coil and a stator core. When the coil is energized an electromagnetic force is produced on an armature. The armature is biased by a return spring and the armature is coupled with a cylinder valve of the engine. The armature is held at the stator core in one operating position of the actuator and, by deenergizing the coil, the armature may move towards and into another operating position by the return spring.
In the above described high speed electromagnetic actuator, a relatively long delay in release of the armature from the stator core may occur due to the time required to dissipate the magnetic field required to generate a holding force on the armature. Further, the time of actual break-away of the armature from the stator core is delayed due to mechanical sticking of the armature/stator core interfaces, enhanced by the presence of oil, and further delayed, in the case of actuators for exhaust valves, by exhaust back pressure, which must be overcome to open the valve. These conditions may cause limitations on high speed and high engine load operation by limiting the maximum rpm achievable.
While measured in small fractions of a second, these delays can be significant in electromagnetic actuators, since in order for the engine to reach high revolutions per minute, fractions of a millisecond are important in operation of the actuator.
Attempts have been made to alleviate the mechanical "sticking" of the armature at a stator core. For example, increasing spring rates of springs acting on the armature have been proposed, but this proposal can lead to an unacceptable size of the actuator and increased power requirements of the actuator magnetic circuit.
Thus, there is a need to move an armature of an electromagnetic actuator from engagement with a stator core with high force and within milliseconds.